1. Field of the Invention
The present invention relates, in general, to a method for fabricating a surface acoustic wave (SAW) filter package and, in particular, to a method for simply fabricating a structurally stable surface acoustic wave filter package with the use of a top molding process, in which a metal shield layer is formed by a spray process.
2. Description of the Prior Art
Surface acoustic wave filters (hereinafter referred to as “SAW filters”) are frequently used in commercial and other applications as RF and IF filters to provide frequency selectivity and other electronic functions. Because the acoustic wave in the SAW filter often propagates along or very near the surface, the SAW filter is generally very sensitive to surface conditions. Therefore, when the surface acoustic wave filter is packaged by mounting it on a substrate, a protector is provided to the lower side of the SAW filter to form an air gap for protecting the surface of the SAW filter. To secure a resistance to external effects, in particular, on a substrate is mounted the protector-attached SAW filter chip along the outer side of which a metal shield layer is provided. As a result, a SAW filter chip package is obtained.
In order to better understand the background of the present invention, a description will be given of a conventional method for fabricating the surface acoustic wave filter chip package, below.
FIGS. 1a to 1f are schematic cross sectional views illustrating stepwise conventional fabrication of a SAW filter chip package.
With reference to FIG. 1a, the SAW filter chip 3, to a lower side of which protectors 5 are attached, and a substrate 2, to an upper side of which bumps 4 for a flip chip bonding are attached, are provided.
Referring to FIG. 1b, the SAW filter chip 3 is mounted on the substrate 2, and the SAW filter chip is electrically and mechanically connected to a wiring portion of the substrate 2 by the flip chip bonding.
Turning now to FIG. 1c, underfills 6 are filled into a space between the substrate and the SAW filter chip. When underfills 6 are filled between the substrate and the SAW filter chip, an active region positioned on a lower side surface of the SAW filter chip is protected by the air gap formed by protectors 5.
Therefore, the metal shield layer should be formed above an outer wall of the SAW filter chip 3 in order to secure a reliability of the SAW filter chip by intercepting electrical effects from outside the SAW filter chip package. However, a metal layer is difficult to be uniformly formed on a whole outer wall of the SAW filter chip because of stepped side formed at side portions of the SAW filter chip 3 and the substrate 3. Accordingly, a fillet 7 is formed in order to convert the stepped sides having a steep gradient to stepped sides having a gentle gradient, as shown in FIG. 1d. The fillet 7 is composed of an insulating material, and gives the stepped side having the gentle gradient to the flank of the SAW filter chip 3 so that the metal layer can be easily formed on the SAW filter chip.
Referring to FIG. 1e, the metal shield layer 8 is formed on the outer wall of the SAW filter chip. At this time, at least two metal shield layers 8 should be formed. In other words, in order to secure a reliability of the SAW filter chip, an inner metal layer intercepting electrical effects from outside the SAW filter chip package is formed, and then an outer metal layer for preventing oxidation of the inner metal layer owing to exposure of the inner metal layer to the atmosphere is additionally formed on the inner metal layer.
Meanwhile, conventionally, the fillet should be thinly formed because a size of the fillet depends on the size of the SAW filter chip package. The thin fillet, even if formed, cannot act as a complete step-coverage around the flank of the chip, as shown in FIG. 1d. Therefore, a the metal shield layer must be deposited thickly over SAW filter chip lest any area is uncoated with the metal shield layer owing to the step.
After forming of the metal shield layer 8 is accomplished, the resulting SAW filter chip package is marked to allow its identification. That is to say, a dark color paste is coated on the metal shield layer 8 to form an identification layer 9, as shown in FIG. 1f. 
As described above, according to the conventional method for fabricating the SAW filter chip package, the metal shield layer should be uniformly formed on the whole outer wall of the SAW filter chip package because the metal shield layer is formed as an outermost layer of the SAW filter chip package. Accordingly, a step of fillet forming is required for preventing a portion of the outer wall of the SAW filter chip package from being not coated owing to the stepped sides formed by the chip and the substrate. Furthermore, the metal shield layer is exposed to the atmosphere as the outermost portion of the SAW filter chip package, and secures a reliability of the SAW filter chip by intercepting electrical effects from outside the SAW filter chip package. Therefore, a further outer layer for preventing oxidation of the metal shield layer should be formed.
Accordingly, the conventional method for fabricating the SAW filter chip package has disadvantages in that the step of the fillet forming is additionally required and at least two metal shield layers having a sufficient thickness are formed on the outer wall of the SAW filter chip, and so productivity of a process for fabricating the SAW filter chip package is very poor.
There is another disadvantage of the conventional method for fabricating the SAW filter chip package in that a thin package substrate or the flank of the SAW filter chip is structurally breakable by a dropping of the SAW filter chip package or application of an external pressure.
Therefore, there continues to be a need for an improved method for simply fabricating a stable surface acoustic wave filter package.